Physics in Your World

These faces were photographed in infrared light. -- Notice how different parts of the face are brighter or dimmer, and compare the two noses. -- Infrared images are sensitive to the temperature of different parts of the face because the heat radiation emitted per second varies as the fourth power of the temperature, so a small difference in temperature makes a big difference in the brightness of the image. -- For more on this temperature dependence, see Hyperphysics' Cooling of the Human Body. For related images, see Caltech's Our IR World Gallery

Physics at Home

In this Exploratorium activity, you focus a beam of invisible heat radiation on your skin with a large parabolic mirror and then use your skin as a radiation detector. To learn more about this activity, visit Exploratorium Science Snacks: Hot Spot, and while you're there, notice the image of the radiant heater made by the mirror in visible light.

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From Physics Research

This image shows a quartz liner in an oven at 1150° C, and the orange glow is heat radiation. -- This radiation is much brighter than that given off by a face (see Physics in Your World) and has its maximum brightness at a much higher frequency--in the visible spectrum rather than infrared. -- For another example of a substance at 1100°-1200° C giving off heat radiation, see the San Diego State University page on basaltic lava.

Worth a Look

"Blackbody radiation" is radiation emitted by a body that is in equilibrium with its own radiation.-- 19th-century physics was only partially successful in explaining blackbody radiation, and quantum ideas were required to achieve full agreement between theory and experiment. -- For an introduction, see Blackbody Radiation, and for more on the physics, try the Hyperphysics page Blackbody Radiation.